The light of the point light source is diffused outward in the shape of a sphere. As the diffusion distance increases, the amount of light energy per unit surface area of the sphere becomes smaller and smaller.
How to calculate the light energy?
In the figure, the unit light energy of a sphere at radius 1 is set to , and at radius r is set to .
the law of conservation of energy, the total energy is constant. So we get the equation